Mn-Zn ferrite and production process thereof

ABSTRACT

The present invention provides a Mn—Zn ferrite having an electrical resistivity exceeding 1 Ωm order and having a high initial permeability of 4000 or more at 100 kHz and 100 or more at 10 MHz. The main components of the Mn—Zn ferrite include 44.0 to 49.8 mol % Fe 2 O 3 , 15.0 to 26.5 mol % ZnO, 0.1 to 3.0 mol % CoO, 0.02 to 1.00 mol % Mn 2 O 3 , and the remainder MnO. The Mn—Zn ferrite can be used in a wide frequency region of 100 kHz to 10 MHz by limiting Fe 2 O 3  content to a range of less than 50 mol %, that is the stoichiometric composition, inhibiting formation of Mn 2 O 3  and adding a proper amount of CoO.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an oxide magnetic materialhaving soft magnetism, and more particularly relates to a Mn—Zn ferritesuitable for use as various inductance elements, impedance elements forEMI countermeasures and the like, and to a production process thereof.

[0003] 2. Description of the Related Art

[0004] Typical oxide magnetic materials having soft magnetism include aMn—Zn ferrite. Conventionally, this Mn—Zn ferrite usually has a basiccomponent composition containing 52 to 55 mol % Fe₂O₃ on the averageexceeding 50 mol %, which is the stoichiometric composition, 10 to 24mol % ZnO and the remainder MnO. The Mn—Zn ferrite is usually producedby mixing respective material powders of Fe₂O₃ZnO and MnO in aprescribed ratio, subjecting mixed powders to respective steps ofcalcination, milling, component adjustment, granulation and pressing toobtain a desired shape, then conducting sintering treatment at 1200 to1400° C. for 2 to 4 hours in a reducing atmosphere in which a relativepartial pressure of oxygen is controlled to a low level by supplyingnitrogen. The reason why the Mn—Zn ferrite is sintered in the reducingatmosphere is that Fe²⁺ is formed as the result of reducing a part ofFe³ ⁺. This Fe2+ has positive crystal magnetic anisotropy and cancelsout negative crystal magnetic anisotropy of Fe³ to thereby enhance softmagnetism.

[0005] Amount of the above-mentioned Fe²⁺ formed depends on relativepartial pressures of oxygen in sintering and cooling after thesintering. Therefore, when the relative partial pressure of oxygen isimproperly set, it becomes difficult to ensure excellent soft magneticproperties. Thus, conventionally, the following expression (1) has beenexperimentally established and the relative partial pressure of oxygenin sintering and in cooling after the sintering has been controlledstrictly in accordance with this expression (1).

log Po ₂=−14540/(T+273)+b  (1)

[0006] where T is temperature (° C.), Po₂ is a relative partial pressureof oxygen, and b is a constant, which is usually set at 7 to 8. The factthat the constant b is 7 to 8 means that the relative partial pressureof oxygen in the sintering must be controlled in a narrow range, whichmakes the sintering treatment very troublesome, thereby increasingproduction costs.

[0007] Additionally, in recent years, with miniaturization andperformance improvement of electronic equipments there is an increasingtendency that signals are processed at a higher frequency. Thus, amagnetic material having excellent magnetic properties even in a higherfrequency region as well has been needed.

[0008] However, when the Mn—Zn ferrite is used as a magnetic corematerial, an eddy current flows in a higher frequency region appliedresulting in a larger loss. Therefore, in order to extend an upper limitof the frequency at which the Mn—Zn ferrite can be used as a magneticcore material, an electrical resistivity thereof must be made as high aspossible. However, since the above-mentioned general Mn—Zn ferritecontains Fe₂O₃ in an amount larger than 50 mol % which is thestoichiometric composition, a large amount of Fe 2+ ions are present,thereby making the transfer of electrons between the above-mentionedFe3+ and Fe²⁺ ions easy. Thus, the electrical resistivity of the Mn—Znferrite is in the order of 1 Ωm or less. Accordingly, an applicablefrequency is limited to about several hundreds kHz at highest, and in afrequency region exceeding the limit, permeability (initialpermeability) is significantly lowered to take away properties of thesoft magnetic material.

[0009] In order to increase an apparent resistance of the Mn—Zn ferrite,in some cases, CaO, SiO₂ or the like is added as additive to impart ahigher resistance to grain boundaries and at the same time the Mn—Znferrite is sintered at as low as about 1200° C. to diminish the grainsize from its usual dimension, about 20 μm, to 5 μm, which constitutesmeasures to increase the ratio of the grain boundary. However, even ifsuch measures are adopted, it is difficult to obtain an electricalresistivity exceeding 1 Ωm order as the grain itself has a lowresistance, and the above-mentioned measures fall short of a thoroughsolution.

[0010] Further, a Mn—Zn ferrite to which, for example, CaO, SiO₂, SnO₂and TiO₂ are added to obtain a higher resistance has been developed andis disclosed in Japanese Patent Application No. Hei 9-18092. However,the electrical resistivity of the Mn—Zn ferrite is as low as 0.3 to 2.0Ωm, which is insufficient for application in a high frequency region.Further, a Mn—Zn ferrite containing 50 mol % or less Fe₂03 to which SnO₂or the like is added is disclosed in EPC 1,304,237. Although it issupposedly very difficult for Fe²⁺ to be formed when Fe₂O₃ content is 50mol % or less, the Mn—Zn ferrite described in this EPC patent containsas much as 3 to 7 mol % Fe²⁺. Therefore, the electrical resistivity ofthe Mn—Zn ferrite in the EPC patent cannot exceed the electricalresistivity of a conventional general Mn—Zn ferrite.

[0011] On the other hand, a Mn—Zn based ferrite which contains less than50 mol % Fe₂O₃ for a higher resistance has been developed for use as acore material for a deflecting yoke and is disclosed in Japanese PatentLaid-Open Nos. Hei 7-230909, Hei 10-208926, Hei 11-199235 and the like.

[0012] However, judging from the fact that the application thereof is acore material for a deflecting yoke and from examples of the inventiondescribed in each publication, the Mn—Zn based ferrites described in anyof the above publications are ferrite materials intended to be appliedin a frequency region of 64 to 100 kHz. It is described that reason forsetting the Fe₂O₃ content to 50 mol % or less for obtaining a highresistance is to make it possible to wind a copper wire directly arounda core for a deflecting yoke. Thus, those publications do not suggestthe application of the Mn—Zn based ferrite in such a high frequencyregion as exceeding 1 MHz. All the Mn—Zn based ferrites have an initialpermeability of about 1100 at 100 kHz, and excellent soft magneticproperties cannot be obtained by merely setting the Fe₂O₃ content toless than 50 mol % so as to obtain a high electrical resistivity.

[0013] Further, Japanese Patent Examined Application No. Sho 52-4753discloses a Mn—Zn ferrite containing 50 mol % or less Fe₂O₃, to which1.3 to 1.5 mol % CoO was added in order to decrease the temperaturecoefficient of initial permeability. This Mn—Zn ferrite also contains aslow as 11 mol % ZnO and its relative partial pressure of oxygen atsintering and cooling is not strictly controlled. Thus, the initialpermeability at 100 kHz is about 2000.

SUMMARY OF THE INVENTION

[0014] The present invention has been made in consideration of theabove-mentioned conventional problems, and an object of the presentinvention is therefore to provide a Mn—Zn ferrite which has a higherelectrical resistivity than 1 Ωm order and at the same time high initialpermeabilities of 4000 or more at 100 kHz and of 100 or more at 10 MHz,and a production process by which such a Mn—Zn ferrite can be obtainedeasily and inexpensively.

[0015] A Mn—Zn ferrite according to an aspect of the present inventionto attain the above-mentioned object is characterized in that the maincomponents include 44.0 to 49.8 mol % Fe₂O₃, 15.0 to 26.5 mol % ZnO, 0.1to 3.0 mol % CoO, 0.02 to 1.00 mol % Mn₂O₃ and the remainder MnO.

[0016] The present Mn—Zn ferrite may contain, in addition to theabove-mentioned main components, at least one of 0.010 to 0.200 mass %V₂O₅, 0.005 to 0.100 mass % Bi₂O₃, 0.005 to 0.100 mass % In₂O₃, 0.005 to0.100 mass % PbO, 0.001 to 0.100 mass % MoO₃ and 0.001 to 0.100 mass %WO₃ as additive.

[0017] Further, the present Mn—Zn ferrite has an initial permeability of4000 or more at 100 kHz and 100 or more at 10 MHz at room temperature(25° C.).

[0018] Still further, a production process according to the presentinvention to attain the above-mentioned object is characterized in thata mixed powder whose components are adjusted so as to obtain thecomposition of the above-mentioned Mn—Zn ferrite is pressed, thensintered and cooled, after the sintering, down to 500° C. or lower in anoxygen atmosphere with a relative partial pressure of oxygen defined byan arbitrary value selected from a range of 6 to 10 as a constant b inthe expression (1).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] In a conventional general Mn—Zn ferrite, Fe₂O₃ content is morethan 50 mol % that is the stoichiometric composition, as describedabove. Thus, in order to prevent this excessive Fe₂O₃ from gettingprecipitated as hematite, sintering and cooling had to be performedunder a condition where a relative partial pressure of oxygen is reducedto a significantly lower level by flowing nitrogen, that is a conditionobtained with the constant b set to 7 to 8. On the other hand, since theMn—Zn ferrite of the present invention contains less than 50 mol %Fe₂O₃, hematite is hardly precipitated. Thus, even if a range ofrelative partial pressure of oxygen in sintering is somewhat increased,excellent magnetic properties can be obtained. Further, in aconventional Mn—Zn ferrite that contains more than 50 mol % Fe₂O₃, about3.0 mol % Fe²⁺ exists. On the other hand, in the Mn—Zn ferrite of thepresent invention, Fe 2+ content is as little as 0.1 to 0.7 mol %.Accordingly, the electrical resistivity of the Mn—Zn ferrite of thepresent invention is very high. Therefore, an eddy current is notincreased so much even in a high frequency region, and an excellentinitial permeability can be obtained. However, if this Fe₂O₃ content istoo small, the saturation magnetization is deteriorated. Thus, at least44.0mol % Fe₂O₃ is needed.

[0020] ZnO as main component influences the Curie temperature and thesaturation magnetization. Too small amount of ZnO reduces the initialpermeability, but on the contrary, too large amount of ZnO lowers thesaturation magnetization and the Curie temperature, so ZnO content isset to the above-mentioned range of 15.0 to 26.5 mol %.

[0021] Since Co²⁺ in CoO has a positive crystal magnetic anisotropy, CoOcan cancel out a negative crystal magnetic anisotropy of Fe³⁺ even ifFe²⁺ having a positive crystal magnetic anisotropy exists only in asmall amount. Further, Co²⁺ has an effect of reducing the loss in a highfrequency region by generating an induction magnetic anisotropy.However, when CoO content is too small, the effect is small. On thecontrary, when the CoO content is too large, the magnetic strain becomeslarge and the initial permeability is decreased. Thus, the CoO contentis set to 0.1 to 3.0 mol %.

[0022] A manganese component in the above-mentioned ferrite exists asMn²⁺ and Mn³⁺. However, since Mn³⁺ strains a crystal lattice, therebysignificantly lowering the initial permeability, Mn₂O₃ content is set to1.00 mol % or less. However, if the Mn₂O₃ content is too small, theelectrical resistivity is significantly decreased. Thus, at least 0.02mol % Mn₂O₃ is to be contained in the ferrite.

[0023] In the present invention, at least one of V₂O₅, Bi₂O₃, In₂O₃,PbO, MoO₃ and WO₃ can be contained as additive(s). All of theseadditives have an action of accelerating grain growth. The initialpermeability in a comparatively lower frequency region depends on itsgrain size, so the initial permeability in a lower frequency region canbe enhanced by allowing the above-mentioned additive(s) to be contained.However, if the content thereof is too small, the effects are small. Onthe contrary, if the content is too large, grains grow abnormally.Accordingly, it is desirable that V₂O₅ be set to 0.01 to 0.200 mass %,Bi₂O₃, In₂O₃ and PbO be respectively set to 0.005 to 0.100 mass %, andMoO₃ and WO₃ be respectively set to 0.001 to 0.100 mass %.

[0024] In the present invention, as described above, amount of Mn3+ iscontrolled by conducting sintering and cooling after the sintering in anatmosphere of a relative partial pressure of oxygen obtained by using anarbitrary value in a range of 6 to 10 as a constant b in the expression(1). When a value larger than 10 is selected as the constant b, theamount of Mn³⁺ in the ferrite becomes larger than 1 mol % whereby theinitial permeability is rapidly lowered. Therefore, the amount of Mn³⁺in the ferrite must be decreased to increase the initial permeability.Thus, it is desirable that a small value be selected as the constant b.However, when a value smaller than 6 is selected, amount of Fe 2+ isincreased or amount of Mn³⁺ is decreased too much whereby the electricalresistivity is significantly lowered. Accordingly, the constant b is setto at least 6.

[0025] In production of the Mn—Zn ferrite, respective raw materialpowders of Fe₂O₃, ZnO, CoO, Mn₂O₃ and MnO, which are the maincomponents, are previously weighed for a prescribed ratio, and mixed toobtain a mixed powder, and then this mixed powder is calcined and finelymilled. Although temperatures for the calcination may be set slightlydifferent from each other depending on target compositions, appropriatetemperatures are selected from a range of 800 to 1000° C. Further, ageneral-purpose ball mill can be used for the fine milling of thecalcined powder. When V₂O₅ , Bi₂O₃, In₂O₃, PbO, MoO₃ and WO₃ are made tobe contained as additives, proper amounts of powders of these additiveare added to the fine milled powder and mixed with each other to obtaina mixture having a target composition. Then, in accordance with a usualferrite production process, granulation and pressing are conducted, andsintering is then conducted at 1100 to 1400° C. Incidentally, in thegranulation a process of adding a binder such as polyvinyl alcohol,polyacrylamide, methyl cellulose, polyethylene oxide or glycerin can beused, and in the pressing a process of applying pressure of, forexample, 80 MPa or more can be used.

[0026] In the above-mentioned sintering and cooling after the sintering,a relative partial pressure of oxygen is controlled by flowing inert gassuch as nitrogen gas or the like into a sintering furnace. In this case,as the constant b in the expression (1), an arbitrary value can beselected from a range of 6 to 10. Thus, the constant b has a very wideallowance as compared to the constant b (7 to 8) selected in a casewhere a conventional general Mn—Zn ferrite containing more than 50 mol %Fe₂O₃ is sintered, and the relative partial pressure of oxygen can beeasily controlled. Further, in this case, the cooling after thesintering needs to be performed in accordance with the above-mentionedexpression (1) only until the temperature gets down to 500° C. becausethe reaction of oxidation or reduction at a temperature lower than 500°C. can be ignored irrespective of the relative partial pressure ofoxygen.

EXAMPLES Example 1

[0027] Respective raw material powders of Fe₂O₃ CoO, MnO, Mn₂O₃ and ZnOwere weighed for a composition of 42.0 to 51.0 mol % Fe₂O₃, 0 to 4 mol %CoO, and the remainder including MnO, Mn₂O₃ and ZnO with a molar ratioof MnO to ZnO being 3:2 when both MnO and Mn₂O₃ are all counted as MnO,and mixed with a ball mill. Then, the mixed powder was calcined in theair at 900° C. for 2 hours and milled with the ball mill for 20 hours,and a fine milled powder was obtained. Then, this fine milled powder wasadjusted with regard to the component so as to obtain the compositionabove mentioned and further mixed with the ball mill for 1 hour. Then,this mixture was granulated with addition of polyvinyl alcohol andpressed at a pressure of 80 MPa into toroidal cores (green compacts)each having an outer diameter of 18 mm, an inner diameter of 10 mm and aheight of 4 mm. Then, the green compacts were placed in a sinteringfurnace where an atmosphere was adjusted by flowing nitrogen so as tohave such a relative partial pressure of oxygen as obtained by settingthe constant b in the expression (1) to 8, sintered at 1300° C. for 3hours and cooled after the sintering, and samples 1-1 to 1-9 as shown inTable 1 were obtained.

[0028] Final component compositions of the respective samples 1-1 to 1-9thus obtained were checked by a fluorescent X ray analysis, and theirelectrical resistivities and initial permeabilities at 100 kHz and 10MHz were measured. The results are shown together in Table 1. TABLE 1Electrical Sample Main Component (mol %) Resistivity InitialPermeability No. Classification Fe₂O₃ MnO* ZnO CoO (Ωm) 100 kHz 10 MHz1-1 Comparison 51.0 29.3 19.6 0.1 0.2 3190  1 1-2 Comparison 50.2 29.819.9 0.1 0.5 3660  1 1-3 Present Invention 49.8 30.1 20.0 0.1  80 4080110 1-4 Present Invention 49.0 30.3 20.2 0.5 110 4260 140 1-5 Comparison47.0 31.8 21.2 0 170 1980  50 1-6 Present Invention 47.0 31.2 20.8 1.0140 4400 180 1-7 Comparison 47.0 29.4 19.6 4.0 150 2760  80 1-8 PresentInvention 44.0 31.8 21.2 3.0 230 4030 170 1-9 Comparison 42.0 33.6 22.42.0 270 2380  90

[0029] As apparent from the results shown in Table 1, all the samples1-3 to 1-9 each containing less than 50 mol % Fe₂O₃ have significantlyhigher electrical resistivities than the comparative samples 1-1 and 1-2each containing more than 50 mol % Fe₂O₃. Further, out of these samples,the samples 1-3, 1-4, 1-6 and 1-8 of the present invention containing44.0 to 49.8 mol % Fe₂O₃ and 0.1 to 3.0 mol % CoO obtained the initialpermeabilities of 4000 or more at 100 kHz and 100 or more at 10 MHz aswell.

Example 2

[0030] Respective raw material powders of Fe₂O₃, CoO, ZnO, MnO and Mn₂O₃were weighed for a composition of 48.0 mol % Fe₂O₃, 1.0 mol % CoO, 12.0to 27.0 mol % ZnO, and the remainder including MnO and Mn₂O₃, and mixedwith a ball mill, and samples 2-1 to 2-6 shown in Table 2 were obtainedby following the same production conditions as employed in Example 1.Final component compositions of the respective samples 2-1 to 2-6 thusobtained were checked by a fluorescent X ray analysis, and their initialpermeabilities at 100 kHz and 10 MHz and Curie temperatures weremeasured. The results are shown together in Table 2. TABLE 2 InitialCurie Sample Main Component (mol %) Permeability Temperature No.Classification Fe₂O₃ MnO* ZnO CoO 100 kHz 10 MHz (° C.) 2-1 Comparison48.0 39.0 12.0 1.0 2960 190 190 2-2 Present Invention 48.0 36.0 15.0 1.04010 180 180 2-3 Present Invention 48.0 33.0 18.0 1.0 4330 170 150 2-4Present Invention 48.0 30.0 21.0 1.0 4600 160 130 2-5 Present Invention48.0 27.0 24.0 1.0 4430 140 100 2-6 Comparison 48.0 24.0 27.0 1.0 4220130  70

[0031] As can be seen from the results shown in Table 2, the initialpermeabilities of 4000 or more at 100 kHz and 100 or more at 10 MHz aswell are obtained on the samples 2-2 to 2-6 containing 15.0 mol % ormore ZnO. However, the comparative sample 2-6 containing 27.0 mol % ZnOhas a low Curie temperature of 70° C., which causes a problem inpractical use.

Example 3

[0032] Respective raw material powders were weighed for the samecomposition as in the sample 1-6 of Example 1 and mixed with a ballmill. Then, the mixed powder was calcined in the air at 900° C. for 2hours and was further milled with the ball mill for 2 hours to therebyobtain a fine milled powder. Then, this fine milled powder was adjustedwith regard to the component so as to obtain the composition specifiedin the above and further mixed with the ball mill for 1 hour. Then, thismixture was granulated with addition of polyvinyl alcohol and pressed ata pressure of 80 MPa into toroidal cores (green compacts) each having anouter diameter of 18 mm, an inner diameter of 10 mm and a height of 4mm. Then, the green compacts were placed in a sintering furnace where anatmosphere was adjusted by flowing nitrogen so as to have such arelative partial pressure of oxygen as obtained by setting variously theconstant b in the expression (1) to a range of 5.5 to 12, sintered at1300° C. for 3 hours and cooled after the sintering, and samples 3-1 to3-5 as shown in Table 3 were obtained.

[0033] The electrical resistivities and initial permeabilities at 100kHz and 10 MHz of the samples 3-1 to 3-5 thus obtained were measured.Further, quantitative analysis of Mn₂O₃ in the respective samples wasperformed. The results are shown together in Table 3. TABLE 3 ElectricalSample b Resistivity Initial Permeability Mn₂O₃ No. ClassificationConstant (Ωm) 100 kHz 10 MHz (mol %) 3-1 Comparison   5.5  9 4620  700.01 3-2 Present Invention  6  70 4700 120 0.24 3-3 Present Invention  8140 4440 180 0.43 3-4 Present Invention 10 250 4150 180 0.81 3-5Comparison 12 300 2810 190 1.20

[0034] As can be seen from the results shown in Table 3, all of thesamples 3-2 to 3-4 of the present invention which were sintered inatmospheres of relative partial pressures of oxygen obtained by settingthe constant b in the expression (1) to 6 to 10 have high initialpermeabilities at 100 kHz and at 10 MHz. However, since the comparativesample 3-1 which was sintered in an atmosphere of a relative partialpressure of oxygen obtained by setting the constant b to 5.5 has thelowest electrical resistivity, it has the lowest initial permeability at10 MHz. On the contrary, since the comparative sample 3-5 which wassintered by setting the constant b to 12 contains as much as 1.1 mol %Mn₂O₃, it has the lowest initial permeability at 100 kHz.

Example 4

[0035] Respective raw material powders were weighed for the samecomposition as in the sample 1-6 of Example 1 and mixed with a ballmill. Then, the mixed powder was calcined in the air at 900° C. for 2hours and milled with the ball mill for 20 hours to thereby obtain afine milled powder. Then, this fine milled powder was adjusted withregard to the component so as to obtain the composition specified in theabove, and a given amount of V₂O₅, Bi₂O₃, In₂O₃, PbO, MnO₃ or WO₃ wasadded to the fine milled powder as additive, then obtained mixture wasfurther mixed with the ball mill for 1 hour. Then, this mixture wasgranulated with addition of polyvinyl alcohol and pressed at a pressureof 80 MPa into toroidal cores (green compacts) each having an outerdiameter of 18 mm, an inner diameter of 10 mm and a height of 4 mm.Then, the green compacts were placed in a sintering furnace where anatmosphere was adjusted by flowing nitrogen so as to have such arelative partial pressure of oxygen as obtained by setting the constantb in the expression (1) to 8, sintered at 1300° C. for 3 hours andcooled after the sintering, and samples 4-1 to 4-8 as shown in Table 4were obtained. And average grain size and initial permeability at 100kHz of the samples 4-1 to 4-8 thus obtained were measured. The resultsare shown together in Table 4. TABLE 4 Initial Sample Additive GrainSize Permeability No. Classification (mass %) (μm) 100 kHz 1-6 PresentInvention — 14 4400 4-1 Present Invention V₂O₅ 0.010 16 4490 4-2 PresentInvention V₂O₅ 0.200 21 4630 4-3 Comparison V₂O₅ 0.300 Abnormal 2790Grain 4-4 Present Invention Bi₂O₃ 0.050 20 4620 4-5 Present InventionIn₂O₃ 0.050 19 4570 4-6 Present Invention PbO 0.050 17 4580 4-7 PresentInvention MoO₃ 0.050 21 4700 4-8 Present Invention WO₃ 0.050 16 4510

[0036] As can be seen from the results shown in Table 4, all of thesamples 4-1,4-2 and 4-4 to 4-8 of the present invention each containinga proper amount of additive have larger grain sizes and have initialpermeabilities further improved, as compared with the sample 1-6 of thepresent invention containing no additive. However, the comparativesample 4-3 excessively containing V₂O₅ as additive incurs an abnormalgrain growth, which significantly lowers the initial permeability.

[0037] As described above, the Mn—Zn ferrite of the present invention,which contains Fe₂O₃ in an amount of 44.0 to 49.8 mol %, that is lessthan the stoichiometric composition, also contains 0.1 to 3.0 mol % CoOand 0.02 to 1.00 mol % Mn₂O₃, and is sintered in an atmosphere of asuitable relative partial pressure of oxygen, obtains excellent initialpermeabilities in a wide range of 100 kHz to 10 MHz. Thus, the utilityof the Mn—Zn ferrite of the present invention is very large.

[0038] Further, when at least one of V₂O₅, Bi₂O₃, In₂O₃, PbO, MoO₃ andWO₃ is contained in proper amounts as additive, a still higherpermeability can be ensured in a low frequency region and the utility isfurther increased.

[0039] Further, according to the production process of the Mn—Zn ferriteof the present invention, the relative partial pressure of oxygen duringsintering and after sintering needs not be strictly controlled, thuslargely contributing to stabilization and cost reduction in Mn—Znferrite production.

What is claimed is:
 1. A Mn—Zn ferrite, characterized in that maincomponents include 44.0 to 49.8 mol % Fe₂O₃, 15.0 to 26.5 mol % ZnO, 0.1to 3.0 mol % CoO, 0.02 to 1.00 mol % Mn₂O₃ and a remainder MnO.
 2. TheMn—Zn ferrite according to claim 1 , wherein at least one of 0.010 to0.200 mass % V₂O₅, 0.005 to 0.100 mass % Bi₂O₃, 0.005 to 0.100 mass %In₂O₃, 0.005 to 0.100 mass % PbO, 0.001 to 0.100 mass % MoO₃ and 0.001to 0.100 mass % WO₃ is contained as additive.
 3. The Mn—Zn ferriteaccording to claim 1 or 2 , wherein an initial permeability at roomtemperature (25° C.) is 4000 or more at 100 kHz and 100 or more at 10MHz.
 4. A production process of Mn—Zn ferrite, characterized in that amixed powder whose components are adjusted so as to obtain thecomposition of the Mn—Zn ferrite according to any one of claims 1 to 3is pressed, then sintered and cooled, after the sintering, down to 500°C. or lower in an atmosphere of a relative partial pressure of oxygendefined by the following expression: log Po ₂−14540/(T+273)+b where T istemperature (° C.), PO₂ is a relative partial pressure of oxygen, and bis a constant selected from a range of 6 to 10.